The normal sensing of sounds and motion by the inner ear is critically dependent upon the ionic composition of endolymph, the fluid bathing the apical side of the hair cells. Epithelial cells responsible for production and maintenance of endolymph have previously been studied almost exclusively by observations of transepithelial events (e.g., ion fluxes, transepithelial voltage and resistance), and by measurements of membrane potential and cell volume. These measurements have pointed to the existence of many intracellular regulatory mechanisms which are fundamental to the normal function of these cells and the normal function of hearing and balance. Data on two cell types in particular, dark cells and transitional cells, suggest their involvement in the homeostasis of mono- and di-valent cations in endolymph. The studies proposed in this application are aimed at understanding the mechanisms involved in the regulation of ion transport in the inner ear. The primary technique to be utilized is quantitative microfluorimetry, a technique which allows the measurement of intracellular constituents with fluorescent probes.